This invention relates to the preparation of ceramic materials from polycarbosilanes with increased ceramic yields. The ceramic materials of this invention are obtained by firing a mixture of a polycarbosilane and iron octoate to an elevated temperature in an inert atmosphere or in a vacuum. The iron octoate additives allow for the formation of ceramic materials with increased ceramic yield.
Ceramic material prepared from polycarbosilane polymers are known in the art. Verbeck et al. in German Application Publication No. 2,236,078.7, which is hereby incorporated by reference, prepared ceramic materials by firing at elevated temperatures in an inert atmosphere, a polycarbosilane prepared by the pyrolysis of monosilanes.
Yajima et al. in U.S. Pat. Nos. 4,052,430 (Oct. 4, 1977) and 4,100,233 (July 11, 1978), which are both hereby incorporated by reference, prepared ceramic materials by the pyrolysis of polycarbosilanes in an inert atmosphere or in a vacuum at an elevated temperature. The polycarbosilanes were prepared by thermally decomposing and polycondensing polysilanes.
Yajima et al. in U.S. Pat. Nos. 4,220,600 (Sept. 2, 1980) and 4,283,376 (Aug. 11, 1981) which are both hereby incorporated by reference, prepared ceramic materials by the pyrolysis of polycarbosilanes partly containing siloxane bonds at an elevated temperature under an inert atmosphere or a vacuum. The polycarbosilane partly containing siloxane bonds was prepared by heating polysilanes in the presence of about 0.01 to 15 weight percent of a polyborosiloxane in an inert atmosphere.
Iwai et al. in U.S. Pat. No. 4,377,677 (Mar. 22, 1983), which is hereby incorporated by reference, also produced ceramic materials by the pyrolysis of polycarbosilane at elevated temperatures under an inert atmosphere or vacuum. The polycarbosilanes of Iwai et al. were prepared by heating a polysilane at 50.degree.-600.degree. C. in an inert gas, distilling out a low molecular weight polycarbosilane fraction and then polymerizing the distilled fraction at 250.degree. to 500.degree. C. in an inert atmosphere.
Schilling et al. in U.S. Pat. No. 4,414,403 (November 1983), which is hereby incorporated by reference, produced ceramic material by the pyrolysis of branched polycarbosilanes at elevated temperatures under an inert atmosphere or vacuum. The branched polycarbosilanes were prepared by reacting monosilanes with an active metal in an inert solvent at elevated temperatures where at least some of the monosilanes contain vinyl groups or halomethyl groups capable of forming branching during the polymerization.
Koga et al. U.S. Pat. Nos. 4,105,455 (Aug. 8, 1978) and 4,374,793 (Feb. 22, 1983) disclosed a method of preparing dense sintered silicon carbide bodies from polycarbosilanes which optionally contained sintering aids. The method of U.S. Pat. No. 4,105,455 involves (A) grinding the polycarbosilane to a powder, and (B) hot pressing the polycarbosilane powder so as to produce a dense sintered silicon carbide body by charring the powder in a hot press mold, heating the powder in a monoxidizing atmosphere to decompose the powder to produce silicon carbide, and sintering the silicon carbide under pressure. Sintering aids may be added either during the preparation of the polycarbosilane or may be added to the polycarbosilane powder in the method of U.S. Pat. No. 4,105,455. The method of U.S. Pat. No. 4,374,793 involves (A) grinding the polycarbosilane to a powder, (B) decomposing the polycarbosilane powder thermally to obtain silicon carbide, (C) molding the silicon carbide powder and (D) sintering the molded silicon carbide powder. Sintering aids may be added during the preparation of the polycarbosilane or may be added to the silicon carbide powder. Disclosed sintering aids include the substances or compounds of such elements as B, Al, Fe, Ga, Ti, W, Mg, Ca, Ni, Cr, Mn, Zr, In, Sc, Be, and the like. Preferred sintering aids include substances or compounds of such elements as B, Al, Fe, W, Mg, Ti and others. The halogen- and organic-substituted compounds are preferred as sintering aids.
Yajima et al., J. Mat. Sci., 13, 2569 (1978) and Hasegawa et al., J. Mat. Sci., 18, 3633 (1983) also discuss polycarbosilanes which are useful as preceramic polymers for preparing silicon carbide ceramics.
What is newly discovered is that iron octoate, when added to polycarbosilanes prior to firing at elevated temperatures, allows for the formation of ceramic materials with a significant increase in ceramic yield relative to ceramic materials fired under the same conditions without the additive.